This invention relates to a surface having a tunable electromagnetic impedance which acts as a reconfigurable beam steering reflector.
Steerable antennas today are found in two common configurations: those with a single feed or reflector that is mechanically steered using a gimbal, and those with a stationary array of electronically phased radiating elements. Both have shortcomings, and the choice of system used is often a tradeoff between cost, speed, reliability, and RF (radio frequency) performance. Mechanically steered antennas are inexpensive, but moving parts can be slow and unreliable, and they can require an unnecessarily large volume of unobstructed free space for movement. Active phased arrays are faster and more reliable, but they are much more expensive, and can suffer from significant losses due to the complex feed structure required to supply the RF signal to and/or receive the RF signal from each active element of the phased array. Losses can be mitigated if an amplifier is included in each element or subarray, but this solution contributes to noise and power consumption and further increases the cost of the antenna.
One alternative is to use a reflectarray geometry, and replace the lossy corporate feed network with a free space feed. The actively phased elements operate in reflection mode, and are illuminated by a single feed antenna. The array steers the RF beam by forming an effective reflection surface defined by the gradient of the reflection phase across the array. Using current techniques, such a system still requires a large number of expensive phase shifters.
There is a need for a reflective surface, in which the reflection phase could be arbitrarily defined, and easily varied as a function of position. The surface should be less expensive than a comparably sized array of conventional phase shifters, yet hopefully offer similar RF performance. Such a surface could behave as a generic reconfigurable reflector, with the ability to perform a variety of important functions including steering or focusing of one or more RF beams. It is the object of this invention to fulfill this need.
The reconfigurable reflector disclosed herein is based a resonant textured ground plane, often known as the high-impedance surface or simply the Hi-Z surface. This electromagnetic structure has two important RF properties that are applicable to low profile antennas. It suppresses propagating surface currents, which improves the radiation pattern of antennas on finite ground planes and it provides a high-impedance boundary condition, acting as an artificial magnetic conductor, which allows radiating elements to lie in close proximity to the ground plane without being shorted out. It has origins in other well-known electromagnetic structures such as the corrugated surface and the photonic band gap surface. A prior art high-impedance surface is disclosed in a pending US patent application of D. Sievenpiper, E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d, U.S. provisional patent application Ser. No. 60/079,953, filed on Mar. 30, 1998.
A prior art high-impedance surface is shown in FIG. 1. It consists of an array of metal top plates or elements 10 on a flat metal sheet 12. It can be fabricated using printed circuit board technology with the metal plates or elements 10 formed on a top or first surface of a printed circuit board and a solid conducting ground or back plane 12 formed on a bottom or second surface of the printed circuit board. Vertical connections are formed as metal plated vias 14 in the printed circuit board, which connect the elements 10 with the underlying ground plane 12. The metal members, comprising the top plates 10 and the vias 14, are arranged in a two-dimensional lattice of cells or cavities, and can be visualized as mushroom-shaped or thumbtack-shaped members protruding from the flat metal surface 12. The thickness of the structure, which is controlled by the thickness of the printed circuit board, is much less than one wavelength for the frequencies of interest. The sizes of the elements 10 are also kept less than one wavelength for the frequencies of interest. The printed circuit board is not shown for ease of illustration.
Turning to FIG. 2, the properties of this surface can be explained using an effective circuit model or cavity which is assigned a surface impedance equal to that of a parallel resonant LC circuit. The use of lumped cavities to describe electromagnetic structures is valid when the wavelength is much longer than the size of the individual features, as is the case here. When an electromagnetic wave interacts with the surface of FIG. 1, it causes charges to build up on the ends of the top metal plates 10. This process can be described as governed by an effective capacitance C. As the charges slosh back and forth, in response to a radio-frequency field, they flow around a long path P through the vias 14 and the bottom metal surface 12. Associated with these currents is a magnetic field, and thus an inductance L. The capacitance C is controlled by the proximity of the adjacent metal plates 10 while the inductance L is controlled by the thickness of the structure.
The structure is inductive below the resonance and capacitive above resonance. Near its resonance frequency,       ω    =          1              LC              ,
the structure exhibits high electromagnetic surface impedance. The tangential electric field at the surface is finite, while the tangential magnetic field is zero. Thus, electromagnetic waves are reflected without the phase reversal that occurs on a flat metal sheet. In general, the reflection phase can be 0, xcfx80, or anything in between, depending on the relationship between the test frequency and the resonance frequency of the structure. The reflection phase as a function of frequency, calculated using the effective medium model, is shown in FIG. 3. Far below resonance, it behaves like an ordinary metal surface, and reflects with a xcfx80 phase shift. Near resonance, where the surface impedance is high, the reflection phase crosses through zero. At higher frequencies, the phase approaches xe2x88x92xcfx80. The calculated model of FIG. 3 is supported by the measured reflection phase, shown for an example structure in FIG. 4.
A large number of structures of the type shown in FIG. 1 have been fabricated with a wide range of resonance frequencies, including various geometries and substrate materials. Some of the structures were designed with overlapping capacitor plates, to increase the capacitance and lower the frequency. The measured and calculated resonance frequencies for twenty three structures with various capacitance values are compared in FIG. 5. Clearly, the resonance frequency is a predictable function of the capacitance. The dotted line in FIG. 5 has a slope of unity, and indicates perfect agreement. The bars indicate the instantaneous bandwidth of the surface, defined by the frequencies where the phase is between xcfx80/2 and xe2x88x92xcfx802.
For a more detailed description and analysis of the high-impedance surface, see D. Sievenpiper, L. Zhang, R. Broas, N. Alexopolous, E. Yablonovitch, xe2x80x9cHigh-Impedance Electromagnetic Surfaces with a Forbidden Frequency Bandxe2x80x9d, IEEE Transactions on Microwave Theory and Techniques, vol. 47, pp. 2059-2074, 1999 and D. Sievenpiper, xe2x80x9cHigh-Impedance Electromagnetic Surfacesxe2x80x9d, Ph.D. dissertation, Department of Electrical Engineering, University of California, Los Angeles, Calif., 1999.
When the resonant cavities are much smaller than the wavelength of interest, the electromagnetic analysis can be simplified by considering them as lumped LC circuits. The proximity of the neighboring metal plates provides capacitance, while the conductive path that connects them provides inductance. The textured ground plane supports an electromagnetic boundary condition that can be characterized by the impedance of an effective parallel LC circuit, given by             Z      s        =                  j        ⁢                  xe2x80x83                ⁢        ω        ⁢                  xe2x80x83                ⁢        L                    1        -                              ω            2                    ⁢          LC                      ,
The sheet inductance is L=xcexct, where xcexc is the magnetic permeability of the circuit board material, and t is its thickness. For a structure with parallel plate capacitors arranged on a square lattice, the sheet capacitance is C=∈A/d, where e is the electric permitivity of the dielectric insulator, and A and d are the overlap area and separation, respectively, of the metal plates.
The surface has a frequency-dependent reflection phase given by   Φ  =      Im    ⁢          {              Ln        ⁡                  (                                                    Z                s                            -              η                                                      Z                s                            +              η                                )                    }      
where xcex7 is the impedance of free space. Far from the resonance frequency, the surface behaves as an ordinary electric conductor, and reflects with a xcfx80 phase shift.
Near the resonance frequency, the cavities interact strongly with the incoming waves. The surface supports a finite tangential electric field across the lattice of capacitors, and the structure has high, yet reactive surface impedance. At resonance, it reflects with zero phase shift, providing the effective boundary condition of an artificial magnetic conductor. Scanning through the resonance condition from low to high frequencies, the reflection phase varies from xcfx80, to zero, to xe2x88x92xcfx80. Thus, by tuning the resonance frequency of the cavities, one can tune the reflection phase of the surface for a fixed frequency.
This tunable reflection phase is the basis of the reconfigurable beam steering reflector disclosed herein. By varying the reflection phase as a function of position across the surface, one can perform a variety of functions. For example, a linear phase gradient is equivalent to a virtual tilt of the reflector. A saw-tooth phase function transforms the surface into a virtual grating. A parabolic phase function can focus a plane wave onto a small feed horn, allowing the flat surface to replace a parabolic dish.
Features of the present invention include:
1. A device with tunable surface impedance;
2. A method for focusing an electromagnetic wave using the tunable surface; and
3 . A method for steering an electromagnetic wave using the tunable surface.
This invention provides a reconfigurable electromagnetic surface which is capable of performing a variety of functions, such as focusing or steering a beam. It improves upon the high-impedance surface, which is the subject of U.S. Provisional Patent Serial No. 60/079,953, to include the important aspect of tunability.
The present invention provides, in one aspect, a tuneable impedance surface for steering and/or focusing a radio frequency beam, the tunable surface comprising: a ground plane; a first plurality of top plates disposed a distance from the ground plane, the distance being less than a wavelength of the radio frequency beam; and a second plurality of top plates disposed a different distance from the ground plane, the second plurality being moveable relative to the first plurality.